Large rigid reflector surfaces such as in the range of one to seven meters in diameter for use in solar collectors and antennas are expensive and difficult to construct with reflective surfaces having accurate focusing characteristics. Therefore, it is cheaper to form the concave reflective shape from a thin membrane of yieldable material.
One method of manufacturing these thin membrane concave reflectors is to use a uniform pressure differential to plastically deform an initially flat membrane to the concave shape by stretching it beyond its elastic limit. This method is disclosed in the U.S. Pat. to Leonhardt et al. No. 4,352,112 issued Sep. 28, 1982. In the Leonhardt patent, it is suggested that an aluminum sheet 0.5 mm to 1 mm thick be initially stretched to a flat shape and then plastically deformed by applying a uniform pressure differential across the sheet. Leonhardt also teaches varying the thickness of the sheet to achieve a more ideal paraboloid shape. After deformation, the deformed sheet is preferably provided with a rigid foam backing. The rigid foam backing allows handling of the fragile membrane without damage when larger paraboloid reflectors are to be constructed. Transportation of a large rigid membrane is impossible. Therefore, the large rigid membrane must be constructed at the erection site.
A 17 meter in diameter, 15 kw solar membrane concentrator has been constructed utilizing stretched metal membrane. See: 50 KW-Solar Membrane Concentrator, Bakr H. Khoshaim. Mr. Khoshaim's report on this project describes the fabrication at pages 3-5+ and 3-39+, as using the steps of welding a plurality of sheet strips together which are 0.5 milli-meters thick and 1.25 meters wide to form a 17 meter in diameter membrane. The membrane is prestretched and then deformed into a paraboloid-like shape by applying a uniform pressure differential across the membrane. Thereafter, glass mirrors 0.6 mm thick are laminated onto the concave surface of the membrane. The entire process is performed on site.
It is believed that the forming of metallic membranes on site as taught in the prior art was a necessity caused by the inability of those involved to solve the problems associated with transporting large concave metallic membranes. Metallic membranes in the range of thickness which are plastically deformable for use as a reflective shape are highly subject to creasing, kinking and deformation during handling. If a concave membrane is creased, kinked or otherwise deformed during handling, it loses its ideal reflective shape and is not an efficient reflector.